It is well known in the art to use topcoats on print paper to protect underlying layers from environmental conditions. In the case of thermal paper, the topcoat serves to protect the underlying functional layer/thermal coating from premature coloration when handling and environmental contaminants. Examples of known topcoats are disclosed in U.S. Pat. Nos. 4,370,370; 4,388,362; 4,424,245; 4,444,819; 4,507,669; 4,551,738 and 4,604,635.
A barrier layer for thermochromic coloring layers comprised of water soluble polymers is described in U.S. Pat. No. 4,370,370. Examples of water-soluble polymers disclosed therein are polyvinyl alcohol, carboxymethyl cellulose, methyl cellulose, ethyl cellulose, hydroxymethyl cellulose. hydroxyethyl cellulose, hydroxypropyl cellulose, polyvinylacrylamide, starch, gelatin casein, and polyvinylpyrolidone. Blends of carboxyl-group modified polyvinyl alcohols in an appropriate solvent with polyamide resin, melamine resin, formalin, glyoxal, or chromium alum are said to provide a barrier layer with high water resistance.
A water-soluble resin solution (polyvinyl alcohol) is used to provide a protective top coating for thermochromic layers in U.S. Pat. No. 4,388,362.
A water-soluble resin mixed with a wax or wax like material is used to form a protective topcoat for thermochromic layers in U.S. Pat. No. 4,424,245. The wax is said to provide improved water resistance. Examples of water-soluble polymers disclosed therein are polyvinyl alcohol, hydroxyethyl cellulose, methyl cellulose, carboxymethyl cellulose, starch and derivatives thereof, casein, sodium alginate, polyvinylpyrolidone, polyacrylamide, and a copolymer of styrene and maleic acid.
Polyvinyl alcohol with a 70-85% saponification ratio is used to form a protective top coat for thermochromic coloring layers in U.S. Pat. Nos. 4,444,819 and 4,551,738. Thermosetting organic fillers, organic antifriction materials and water resistant agents are said to be suitable auxiliary components that can be added to the polyvinyl alcohol. Examples of thermosetting organic fillers are said to be urea-formaldehyde resin, phenolic resin, polycarbonate resin and melamine resin. The organic antifriction agents are waxes that can be used in the form of a fine powder or an emulsion from 20-60 wt. % percent of the protective layer.
Polyvinyl alcohol, cellulose ether, starch, ammonium polycarboxylates and alkaline salts of isobutylene maleic anhydride polymer are described as water soluble resins suitable for forming a protective layer for thermochromic coloring layers in U.S. Pat. No. 4,507,669.
U.S. Pat. No. 4,604,635 discloses a protective coating for thermochromic coloring layers comprised of a cured silicone resin.
These conventional topcoat formulations, excluding the components classified as fillers, generally comprise water-soluble polymers and are not optimized with regard to adhesion to the underlying surface.
There is a continuing need/desire to improve the anchorage of coatings in print media, including topcoats. When improving the anchorage of coatings to the underlying surface on print media, the print quality will generally improve through an improvement in abrasion resistance. A coating's abrasion resistance primarily depends on its mechanical properties but is also related to how well it is anchored to the underlying surface in the case of direct thermal paper, the improved adhesion will improve the abrasion resistance of the topcoat and thereby reduce the abrasion-generated buildup of the topcoat on the thermal print head. By limiting the buildup on the thermal print head, print quality is improved.